EP3217167B1

EP3217167B1 - Humidity sensors with transistor structures and piezoelectric layer

Info

Publication number: EP3217167B1
Application number: EP16160046.5A
Authority: EP
Inventors: Viorel Georgel Dumitru; Viorel Avramescu; Octavian Buiu; Mihai Brezeanu; Bogdan Serban
Original assignee: Honeywell International Inc
Current assignee: Honeywell International Inc
Priority date: 2016-03-11
Filing date: 2016-03-11
Publication date: 2018-05-16
Anticipated expiration: 2036-03-11
Also published as: US10324053B2; EP3217167A1; US20170261453A1

Description

Technical Field

The disclosure relates generally to humidity sensors.

Background

Capacitive and resistive type humidity sensors rely on the ability of the sensing material to quickly absorb and desorb water molecules. The absorbed moisture changes the physical properties of the sensing material either by changing its resistance, permittivity, or stress, which can each directly affect the electrical response of the sensor. Bulk polyimide films are often used as the humidity sensing material in many capacitive and resistive humidity sensors. However, it may be desirable to provide alternative humidity sensors.
Patent document US2005/116263A1 describes multifunctional biological and biochemical sensor technology based on ZnO nanostructures. The ZnO nanotips serve as strong DNA or protein molecule binding sites to enhance the immobilization. Patterned ZnO nanotips are used to provide conductivity-based biosensors. Patterned ZnO nanotips are also used as the gate for field-effect transistor (FET) type sensors. Patterned ZnO nanotips are integrated with SAW or BAW based biosensors. These ZnO nanotip based devices operate in multimodal operation combining electrical, acoustic and optical sensing mechanisms. The multifunctional biosensors can be arrayed and combined into one biochip, which will enhance the sensitivity and accuracy of biological and biochemical detection due to strong immobilization and multimodal operation capability. Such biological and biochemical sensor technology are useful in detection of RNA-DNA, DNA-DNA, protein-protein, protein-DNA and protein-small molecules interaction. It can be further applied for drug discovery, and for environmental monitoring and protection.
Patent document number US4698657A describes a field effect transistor-type sensor which comprises a field effect transistor device incorporated with a sensitive means exhibiting electric variation due to a physical or chemical interaction with the physical quantity to be detected. An auxiliary electrode for the application of a drift-cancellation voltage to the sensitive means is located on the sensitive means, thereby suppressing the influence of impurities and/or ions contained in the sensitive means and/or the interface between the sensitive means and the field effect transistor device, or impurities and/or ions contaminating the device from the external atmosphere during use thereof, on the operation and/or the output of the field effect transistor device, and maintaining the stable output characteristic over a long period of time.
Patent document number US5004700A describes a method of making a humidity sensor which comprises providing a host device constituted by a semi-conductor substrate and a gate insulator of an insulated gate field effect transistor, forming a layer of poly (vinyl) alcohol (PVA) on the exposed surface of the insulator, heat treating the layer to crystallize and stabilize the PVA, and forming a gate electrode on the PVA layer, so that the gate electrode is porous allowing ambient water vapor to be absorbed by the PVA. In response, the PVA undergoes a change of bulk dielectric constant, thereby causing a change in gate capacitance of the transistor resulting in a detectable change of electrical conductivity in the drain source channel.
Patent document number WO2015/015253A1 describes a condensation sensors systems and methods. The methods for forming a condensation sensor include depositing a Ill- nitride on a substrate via sputtering, and implementing conductive contacts on the deposited Ill-nitride via a shadow mask.

Summary

The present invention in its various aspects is as set out in the appended claims.
This disclosure relates generally to humidity sensors and methods for making humidity sensors. In one example, a humidity sensor may include a substrate and a sensing field effect transistor. The sensing field effect transistor may comprise a source formed on the substrate, a drain formed on the substrate, a gate, and a piezoelectric layer disposed over the gate. In some instances, the piezoelectric layer may comprise aluminum nitride.
In another example, a humidity sensor may comprise a substrate, a semi-conductor layer disposed over the substrate, a piezoelectric layer disposed over the semi-conductor layer, a first electrode disposed on the piezoelectric layer, and a second electrode disposed on the piezoelectric layer. In some instances, the piezoelectric layer may comprise aluminum nitride.
An illustrative method of manufacturing a humidity sensor may comprise forming a sensing field effect transistor on a substrate, wherein the sensing field effect transistor comprises a source, a drain, and a gate. A piezoelectric layer may be sputtered over the gate of the sensing field effect transistor. The method may further comprise depositing a source contact electrode over the source of the sensing field
effect transistor and depositing a drain contact electrode over the drain of the sensing field effect transistor.
The preceding summary is provided to facilitate an understanding of some of the features of the present disclosure and is not intended to be a full description. A full appreciation of the disclosure can be gained by taking the entire specification, claims, drawings, and abstract as a whole.

Brief Description of the Drawings

The disclosure may be more completely understood in consideration of the following description of various illustrative embodiments of the disclosure in connection with the accompanying drawings, in which:

Figure 1 is a cross-sectional view of an illustrative humidity sensor, which does not form part of the invention but represents background art;
Figure 2 is a cross-sectional view of another illustrative humidity sensor, which does not form part of the invention but represents background art;
Figure 3 is a cross-sectional view of another illustrative humidity sensor; and
Figure 4 is a cross-sectional view of another illustrative humidity sensor, which does not form part of the invention but represents background art.

Description

The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. References to "over," "under," "top," and "bottom," etc., are relative terms and are made herein with respect to the drawings and do not necessarily correspond to any particular orientation in actual physical space. The description and drawings show several examples that are meant to be illustrative of the claimed disclosure.
Humidity sensors may be used for process control in industrial applications, and for ambient air quality monitoring in homes and offices. Relative humidity (RH) sensors for cell phone and other mobile applications may become an emerging technology, within the existing trend of adding more functions to the portable applications. Most currently used humidity sensors employ either resistive (change of electrical resistance) or capacitive (change of dielectricconstant) effects. The present humidity sensor may detect a change in the humidity level in the environment by a change in the current flowing through the transistor.
Figure 1 is a diagram of an exemplary humidity sensor 10 that may be based on a field effect transistor (FET). The humidity sensor may be formed on a p-type silicon substrate or wafer 12. The sensor 10 may have an n-type source 14, an n-type drain 16, and a gate dielectric layer18 consisting of a thermally grown thin layer of SiO₂ bridging n-type source 14 and n-type drain 16. As will be discussed in more detail below, a piezoelectric aluminum nitride (AlN) 20 layer may be disposed over the gate 18. While the piezoelectric material is described as AlN, it is contemplated that other piezoelectric materials may also be used. The piezoelectric layer 20 may be prepared by sputter a piezoelectric material over the gate 18. The sensor 10 may further incorporate a source contact electrode 22 and a drain contact electrode 24 disposed on the substrate 12 and contacting the piezoelectric layer 20. In some instances, the contact electrodes 22, 24 may be formed from aluminum. However, this is not required. In other embodiments, the electrodes 22, 24 may be formed from other electrically conductive materials such as, but not limited to platinum or gold.
The piezoelectric AlN layer 20 may have an electrical polarization that is a combination of spontaneous and piezoelectric polarization. In stationary environmental conditions, on the top side 20a of the piezoelectric layer 20, the polarization induced bound surface charge is screened by adsorbed charges from environment and charged surface defects. At the bottom side 20b of the piezoelectric layer, the polarization induced bound surface charge is compensated by charged interface defects and, eventually by charge redistribution in the semiconductor (like in HEMT transistors). Therefore, no potential difference exists between top 20a and bottom side 20b of AlN layer 20.
When humidity level changes do occur in the environment, it affects the adsorbed charge (mostly water) that compensates the polarization charge on the top surface 20a of the AlN layer 20, which is exposed to the environment. Therefore, the polarization charge will become, for a given time interval, under or overcompensated by the adsorbed charge, thus generating a net charge. On the bottom side 20b of the piezoelectric material 20 the compensating charge is not influenced by an environmental humidity change, since the water vapors cannot reach the bottom piezoelectric material surface 20b. This will lead to a potential difference between top 20a and bottom 20b side of the AlN layer 20, that will affect the current flowing through the transistor (similar to applying a gate voltage). The change in current may be correlated to the humidity of the surrounding environment.
While only a sensing sensor is illustrated in Figure 1, it is contemplated that the humidity sensor may further include a reference sensor. In some instances, the reference sensor may be provided on a separate wafer, while in other instances the reference sensor may be provided on the same wafer as the sensing sensor. For example, the humidity sensor 10a may include a sensing FET 30 including the same structure as described above with respect to Figure 1. The humidity sensor 10a may further include a reference FET 40. It is contemplated that the general structure of the reference sensor may be similar to the sensing sensor. However, the piezoelectric layer may be absent in the reference sensor. For example, the reference FET 40 may have an n-type source 42, an n-type drain 44, and a gate dielectric layer 46 consisting of a thermally grown thin layer of SiO₂ bridging n-type source 42 and n-type drain 44. The reference FET 40 may further incorporate a source contact electrode 48 and a drain contact electrode 50 disposed on the substrate 12. In some instances, the contact electrodes 48, 50 may be formed from aluminum. However, this is not required. In other embodiments, the electrodes 48, 50 may be formed from other electrically conductive materials such as, but not limited to platinum or gold.
Further, the present humidity sensor may be fabricated using various techniques. For example, phosphorus atoms may be implanted/diffused into a p-type silicon substrate to create n-doped regions forming the sources and drains of the reference and sensing FETs. A gate dielectric layer of silicon dioxide, or other appropriate gate insulator material, may then be thermally grown over the substrate followed by masking and etching to define channel regions. Growth of a thin silicon dioxide gate insulator layer, or other appropriate gate insulator material, may form the gate dielectric layer and assure a good surface state density at the silicon-silicon dioxide interface. The piezoelectric layer may then be sputtered onto the gate layer. Sputtering and patterning of an aluminum layer for contacting the source, drain, and gate electrodes may complete the fabrication of the reference and sensing FETs.
One or more of these steps may be modified if it is desirable to employ FETs implemented using different technologies, for example, n-MOS, p-MOS, CMOS, and so on. Similarly, one or more additional process steps may be employed if it is desirable to fabricate an instrumentation amplifier on the same substrate.
Figure 2 is a diagram of another exemplary humidity sensor 100 that may be based on a field effect transistor (FET). The humidity sensor may be formed on a p-type silicon substrate 112. The sensor 100 may have an n-type source 114, an n-type drain 116, and a gate dielectric layer1 18 consisting of a thermally grown thin layer of SiO₂ bridging n-type source 114 and n-type drain 116. As will be discussed in more detail below, a piezoelectric aluminum nitride (AlN) 120 layer may be disposed over the gate 118. While the piezoelectric material is described as AlN, it is contemplated that other piezoelectric materials may also be used. The piezoelectric layer 120 may be prepared by sputter a piezoelectric material over the gate 118. The sensor 100 may further incorporate a source contact electrode 122 and a drain contact electrode 124 disposed on the substrate 112 and contacting the piezoelectric layer 120. The sensor 100 may be further provided with a gate electrode 126 disposed on the piezoelectric layer 120. In some instances, the contact electrodes 122, 124 and gate electrode 126 may be formed from aluminum. However, this is not required. In other embodiments, the electrodes 122, 124, 126 may be formed from other electrically conductive materials such as, but not limited to platinum or gold.
The piezoelectric AlN layer 120 may have an electrical polarization that is a combination of spontaneous and piezoelectric polarization. In stationary environmental conditions, on the top side 120a of the piezoelectric layer 120 the polarization induced bound surface charge isscreened by adsorbed charges from environment and charged surface defects. At the bottom side 120b of the piezoelectric layer, the polarization induced bound surface charge is compensated by charged interface defects and, eventually by charge redistribution in the semiconductor (like in HEMT transistors). Therefore, no potential difference exists between top 120a and bottom side 120b of AlN layer120.
When humidity level changes do occur in the environment, it affects the adsorbed charge (mostly water) that compensates the polarization charge on the top surface 120a of the AlN layer 120, which is exposed to the environment. Therefore, the polarization charge will become, for a given time interval, under or overcompensated by the adsorbed charge, thus generating a net charge. On the bottom side 120b of the piezoelectric material 120 the compensating charge is not influenced by an environmental humidity change, since the water vapors cannot reach the bottom piezoelectric material surface 120b. This will lead to a potential difference between top 120a and bottom 120b side of the AlN layer 120, that will affect the current flowing through the transistor (similar to applying a gate voltage). The change in current may be correlated to the humidity of the surrounding environment. While only a sensing sensor is illustrated in Figure 2, it is contemplated that the humidity sensor may further include a reference sensor. In some instances, the reference sensor may be provided on a separate wafer, while in other instances the reference sensor may be provided on the same wafer as the sensing sensor. It is contemplated that the general structure of the reference sensor may be similar to the sensing sensor. However, the piezoelectric layer 120 may be absent in the reference sensor. It is contemplated that the humidity sensor 100 maybe manufactured in a similar manner to humidity sensor 10 described above.
Figure 3 is a diagram of another exemplary humidity sensor 200 that may be based on a field effect transistor (FET). The humidity sensor may be formed on a p-type silicon substrate 212. The sensor 200 may have an n-type source 214, an n-type drain 216, and a gate dielectric layer 218 consisting of a thermally grown thin layer of SiO₂ bridging n-type source 214 and n-type drain 216. As will be discussed in more detail below, a piezoelectric aluminum nitride (AlN) 220 layer may be disposed over the gate 218. While the piezoelectric material is described as AlN, it is contemplated that other piezoelectric materials may also be used. The piezoelectric layer 220 may be prepared by sputter a piezoelectric material over the gate 218. The sensor 200 may further incorporate a source contact electrode 222 and a drain contact electrode 224 disposed on the substrate 212 and contacting the piezoelectric layer 220. The sensor 200 may be further provided with a gate electrode 226 disposed on the gate 218 and a supplementary electrode 228 provided on the piezoelectric layer 220. In some instances, the contact electrodes 222, 224, gate electrode 226, and supplementary electrode 228 may be formed from aluminum. However, this is not required. In other embodiments, the electrodes 222, 224, 226, 228 may be formed from other electrically conductive materials such as, but not limited to platinum or gold.
The piezoelectric AlN layer 220 may have an electrical polarization that is a combination of spontaneous and piezoelectric polarization. In stationary environmental conditions, on the top side 220a of the piezoelectric layer 220, the polarization induced bound surface charge isscreened by adsorbed charges from environment and charged surface defects. At the bottom side 220b of the piezoelectric layer, the polarization induced bound surface charge is compensated by charged interface defects and, eventually by charge redistribution in the semiconductor (like in HEMT transistors). Therefore, no potential difference exists between top 220a and bottom side 220b of AlN layer 220.
When humidity level changes do occur in the environment, it affects the adsorbed charge (mostly water) that compensates the polarization charge on the top surface 220a of the AlN layer 220, which is exposed to the environment. Therefore, the polarization charge will become, for a given time interval, under or overcompensated by the adsorbed charge, thus generating a net charge. On the bottom side 220b of the piezoelectric material 220 the compensating charge is not influenced by an environmental humidity change, since the water vapors cannot reach the bottom piezoelectric material surface 220b. This will lead to a potential difference between top 220a and bottom 220b side of the AlN layer 220, that will affect the current flowing through the transistor (similar to applying a gate voltage). The change in current may be correlated to the humidity of the surrounding environment. While only a sensing sensor is illustrated in Figure 3, it is contemplated that the humidity sensor may further include a reference sensor. In some instances, the reference sensor may be provided on a separate wafer, while in other instances the reference sensor may be provided on the same wafer as the sensing sensor. It is contemplated that the general structure of the reference sensor may be similar to the sensing sensor. However, the piezoelectric layer 220 may be absent in the reference sensor. It is contemplated that the humidity sensor 200 may be manufactured in a similar manner to humidity sensor 10 described above.
Figures 1-3 illustrate just some potential configurations of the piezoelectric layer 20, 120, 220 used in combination with a field effect transistor. These structures are not intended to be limiting. It is contemplated that the piezoelectric layer 20, 120, 220 may be used in combination with any know FET, or other transistor technology.
Figure 4 a diagram of another exemplary humidity sensor 300 that may be based on a thin film transistor. The humidity sensor may be formed on any of the known thin film transistors, such as, but not limited to those based on amorphous or polycrystalline Si, ZnO, IGZO, ZTO, InN, AlInN, organic materials, OFET, nanowires, nanotubes, etc. Depending on the thin film technology employed, the sensor 300 may be formed on a rigid or flexible substrate 312. A semi-conductor layer 318 may be disposed over the substrate 312. As will be discussed in more detail below, a piezoelectric aluminum nitride (AlN) 320 layer may be disposed over the semi-conductor layer 318. While the piezoelectric material is described as AlN, it is contemplated that other piezoelectric materials may also be used. The piezoelectric layer 320 may be prepared by sputter a piezoelectric material over the semi-conductor layer 318. The sensor 300 may further incorporate a first contact electrode 322 and a second contact electrode 324. It is contemplated that the electrodes 322, 324 may be in contact with only a portion of the piezoelectric layer. In some instances, the contact electrodes 322, 324 may be formed from aluminum. However, this is not required. In other embodiments, the electrodes 322, 324 may be formed from other electrically conductive materials such as, but not limited to platinum or gold.
The piezoelectric AlN layer 320 may have an electrical polarization that is a combination of spontaneous and piezoelectric polarization. In stationary environmental conditions, on the top side 320a of the piezoelectric layer 320 the polarization induced bound surface charge is screened by adsorbed charges from environment and charged surface defects. At the bottom side 320b of the piezoelectric layer, the polarization induced bound surface charge is compensated by charged interface defects and, eventually by charge redistribution in the semiconductor (like in HEMT transistors). Therefore, no potential difference exists between top 320a and bottom side 320b of AlN layer 320.
When humidity level changes do occur in the environment, it affects the adsorbed charge (mostly water) that compensates the polarization charge on the top surface 320a of the AlN layer 320, which is exposed to the environment. Therefore, the polarization charge will become, for a given time interval, under or overcompensated by the adsorbed charge, thus generating a net charge. On the bottom side 320b of the piezoelectric material 320 the compensating charge is not influenced by an environmental humidity change, since the water vapors cannot reach the bottom piezoelectric material surface 320b. This will lead to a potential difference between top 320a and bottom 320b side of the AlN layer 320, that will affect the current flowing through the transistor (similar to applying a gate voltage). The change in current may be correlated to the humidity of the surrounding environment. While only a sensing sensor is illustrated in Figure 4, it is contemplated that the humidity sensor may further include a reference sensor. In some instances, the reference sensor may be provided on a separate wafer, while in other instances the reference sensor may be provided on the same wafer as the sensing sensor. It is contemplated that the general structure of the reference sensor may be similar to the sensing sensor. However, the piezoelectric layer 320 may be absent in the reference sensor.
The disclosure should not be considered limited to the particular examples described above. Various modifications, equivalent processes, as well as numerous structures to which the disclosure can be applicable will be readily apparent to those of skill in the art upon review of the instant specification.

Claims

A humidity sensor (200) for sensing humidity in an environment, the humidity sensor comprising:
a substrate (212);

a sensing field effect transistor comprising:
a source (214) in the substrate;

a drain (216) in the substrate spaced from the source;

a piezoelectric layer (220) exposed to the environment;

an electrical insulator (218) situated between the piezoelectric layer and the substrate, the electrical insulator and the piezoelectric layer situated between the source and drain; and

a gate electrode (226) situated between the piezoelectric layer and the electrical insulator,

wherein the gate electrode is configured to receive a gate voltage sufficient to create a conduction channel in the substrate between the source and the drain and bias the field effect transistor into its linear region,

wherein the piezoelectric layer is configured to generate a net charge across the piezoelectric layer in response to a change in humidity in the environment, and

wherein the change in the net charge is configured to modulate the conduction channel in the substrate between the source and the drain.
The humidity sensor of claim 1, wherein the piezoelectric layer comprises aluminum nitride (AlN).
The humidity sensor of claim 2, wherein the AlN is sputtered onto the electrical insulator.
The humidity sensor of claim 1, wherein the piezoelectric layer comprises zinc oxide (ZnO).
The humidity sensor of any of claims 1-4, further comprising a gate electrode situated above the piezoelectric layer.
The humidity sensor of claim 1, wherein the gate electrode situated between the piezoelectric layer and the electrical insulator is a floating gate electrode.
The humidity sensor of any of claims 1-4, further comprising a supplementary electrode (228) situated above the piezoelectric layer.
The humidity sensor of any of claims 1-7, wherein the substrate comprises a wafer, and the source corresponds to a first implant region in the substrate and the drain corresponds to a second implant region in the substrate.
The humidity sensor of any of claims 1-8, wherein the substrate comprises a rigid substrate.
The humidity sensor of any of claims 1-7, wherein the substrate comprises a flexible substrate.
The humidity sensor of claim 1, wherein the sensing field effect transistor comprises:
a gate, wherein the gate comprises the electrical insulator, wherein the electrical insulator is a gate oxide layer, and

the gate electrode; and

the piezoelectric layer situated above the gate oxide layer.
The humidity sensor of claim 1, wherein water vapor cannot reach a bottom side of the piezoelectric layer.
The humidity sensor of claim 1, wherein piezoelectric layer is configured to generate the net charge for a given time interval in response to a change in an adsorbed charge on a top surface of the piezoelectric layer.